1. Field of the Invention
This invention relates to a disk drive mechanism in which a disk such as a CD (compact disk) and a DVD (digital video disk) is mounted on a turn table, and turned; and more particularly to a mechanism for suppressing the eccentric vibration of a disk which is turned at high speed by a disk drive mechanism.
2. Related Art
There has been a strong demand for improvement of an access speed to a disk as a disk memory such as a CD-ROM, a CD-R/W and a DVD-ROM has been popularly employed in the art. For instance, research has been conducted on an increase in the rotating speed of a disk memory twenty-four times or thirty-two times as high as the ordinary speed of rotation of the CD. In those cases, the speed (rpm) of the disk is 5000 to 6000 rpm. In the future, it may be necessary to increase the speed of the disk to about 10000 rpm.
However, in the case where, when the disk is turned at high speed as described above, a mass eccentricity causes the following problems. First the placement of the disk on the turn table may be shifted. Also the disk will experience an eccentric vibration. Especially the disk such as a DVD and a CD-ROM which is formed by bonding two disks (front and rear disk pieces) together is liable to suffer from mass eccentricity. On the other hand, according to the present industrial standard, the mass eccentricity up to 1 g.cm is permitted. Hence, when turned at high speed, the disk may suffer from eccentric vibration. If the disk suffers from eccentric vibration as described above, then the optical pickup may be lowered in signal writing and reading accuracy, or the vibration may be transmitted to another mechanism adjacent to the disk drive mechanism, which adversely affects the disk drive operation, or makes a noise. This difficulty may be eliminated by the provision of a disk vibration preventing device in addition to the disk drive mechanism. However, in this case, the device is unavoidably bulky as a whole. Therefore, the provision of a conventional disk vibration preventing device is not practical.
On the other hand, in order to prevent the eccentric vibration of the disk which turns at high speed, a vibration preventing device has been provided. In FIG. 11, reference numeral 104 designates the vibration preventing device. In the device 104, an annular magnet 101 is fixedly mounted on the rotary shaft 100 of an electric motor, and a cylindrical outer wall 102 is provided around the magnet 101, and a plurality of steel balls are provided between the magnet 101 and the outer wall 102. With the vibration preventing device, as the speed of the motor increases, the steel balls 103 are shifted to the outer wall 102 by centrifugal force, and as the speed of the motor becomes higher than the resonance frequency of the device, the steel balls 103 remain stable being shifted in the direction opposite to the direction of mass eccentricity F as indicated by the two-dot chain lines. As a result, the mass eccentricity F and the centrifugal force are canceled out by each other, so that the vibration of the disk is prevented. However, the above-described vibration preventing device 104 suffers from the following problems: In the case where the speed of rotation is high, 5000 rpm or higher, the signal writing and reading operation is lowered in accuracy. Furthermore, in the use of the disk which has no mass eccentricity, the friction and shift of the steel balls 103 in the device 104 increases the eccentricity, which may result in the eccentric vibration.
The disk drive mechanism is known in the art which, in order to concentrically mount the disk on the turn table, has a disk position determining mechanism. However, this method makes the disk drive mechanism bulky and high in manufacturing cost.